1. Field of the Invention
This invention relates to polyimides and, more particularly, to low-toxicity, high-temperature polyimides.
2. Description of the Related Art
The cost and weight of aerospace structures can be significantly reduced by the application of high-performance polymeric composites. By replacing metallic components with composite materials, significant weight and cost savings between 25 and 30 percent can be realized. The advent of high modulus fibers and thermally stable polymers made it possible for the introduction of composites into 316.degree. C. (600.degree. F.) temperature regimes. To this end, researchers at NASA Lewis Research Center developed a high-temperature addition polyimnide called PMR-15, which, over several decades, became the high-temperature matrix resin "work horse" for the aerospace industry.
For composite applications in the range of 250.degree. C. to 325.degree. C. the resin system of choice generally has been the PMR-15 polyimide (polymerization of Monomeric Reactants, 1500 molecular weight). The reactant monomers for PMR-15 consist of the monomethyl ester of 5-norbornene 2,3-dicarboxylic acid (nadic ester, NE), the dimethyl ester of 3,3',4,4'-benzophenonetetracarboxylic acid (BTDE), and 4,4'-methylenedianiline (MDA).
For PMR-type solutions, the number of moles of each monomer can be determined by the following ratio: 2:n:n+1, where 2 represents the moles of endcap, n equals the moles of the dialkyl ester of the aromatic tetracarboxylic acid, and n+1 quantifies the moles of the diarine. For PMR-15, this molar ratio becomes 2 NE: n=2.087 BTDE: n+1=3.087 MDA, which corresponds to a formulated molecular weight of 1500 for the imidized pre-polymer. At this molecular weight, a balance between thermo-mechanical properties and processing characteristics can be achieved.
In the reaction sequence of PMR-15, heating the monomers promotes reaction which forms the linear poly(amic acid) pre-polymer. Further heating converts the amic acid groups into the stable heterocyclic imide rings. Both reactions are considered "condensation" type since methanol and water are released as by-products. In actuality, there are many competing reactions that can occur during the imidization of PMR-15. These reactions are complex and lead to prepolymers that contain a variety of chemical functionality such as imide, amide, ester, and anhydride.
PMR-15 contains 4,4'-methylenedianiline (MDA), a known animal carcinogen, a suspected human carcinogen, and a known kidney and liver toxin. When quantities of PMR-15 prepreg are being manufactured, or when this material is being pr oduced into composite structures, exposure to MDA becomes a serious health hazard.
Thus it has become necessary for the Occupational Safety and Health Administration (OSHA) to issue and enforce very strict regulations regarding worker exposure to MDA. In fact, the permissible (human inhalation) exposure limit (PEL) defined by OSHA has been set at 10 parts/billion per eight hour shift. In most manufacturing facilities, this means dedicated work space in which all personnel must wear disposable booties, coveralls, dust masks, and gloves; and all personnel leaving the facility are required to shower.
Disposal of waste materials resulting from the manufacture of PMR-15 composite components is also a significant problem. Utmost care must be taken in handling uncured PMR-15 waste since it contains substantial quantities of unreacted MDA. Typically, 20-30% of the prepreg material issued to the manufacturing process is discarded.
In addition, waste material is generated during the manufacturing process that is contaminated by small quantities of PMR-15 resin: prepreg backing material, roll cores, kit bags, autoclave debulking materials and other process materials. Personal safety items such as shop coats, gloves, booties, and dust masks add to the disposal problem. Currently these materials are commonly disposed of together either through carefully controlled incineration or by being packed in a drum and shipped to a special hazardous material landfill. These methods of disposal are costly, and the material in the landfill remains indefinitely the responsibility of the generator.
There have been other attempts to circumvent the problems caused by the toxicity of MDA. These range from imidizing the PMR-15, so that the MDA is fully reacted before the product is sold to the composite manufacturer and thus harmless, to changing the chemical composition of the PMR-15 formulation. In fact, most ofthe investigations to date have been motivated by scientists measuring the effects of chemical composition on polymer properties, rather than by considerations of safety.
One example is NASA Langley's LARC-160 which is similar to PMR-15 with the exception that the MDA is replaced with a commercial diamine mixture (Jeffamine). This is said to improve the flow, tack, and drape, but at the expense of glass transition temperature and thermal oxidative stability. Cycom X3009 contains excessive amounts of the so-called bis-nadimnide, the condensation product of MDA and two nadic esters. This makes the material difficult to process and causes excessive micro-cracking on thermal cycling of the composites made from this product.
CPI 2320 from SP Systems, Inc., is based on RP46, and is described as an MDA-free PMR polyimide that is both safe and cost-effective as a replacement for PMR-15. However, the thermal stability of CPI 2320 at 316.degree. C. and 1.03 Mpa air pressure is almost an order of magnitude poorer than that of PMR-15. SP Systems compared the stability of RP46 on T650-35 graphite fiber against PMR-15 on G30-500 graphite fiber and found similar results with poor thermal stability. In addition, subsequent toxicity testing completed by NASA-Lewis Research Center has suggested that the diamine in RP46 is also a potential carcinogen.
Many diamines lacking benzylic hydrogen do not make a stable PMR resin. The only (non-fluorinated) resins made with the usual benzophenonetetracarboxylic dimethyl ester that are comparable to PMR-15 in stability have been made with 1,1-bis(4-aminophenyl)-1-phenyl ethane. However, this diamine is an .alpha.-substituted MDA, and therefore can be expected to be relatively toxic and mutagenic. Most diamines chemically related to MDA (as well as MDA itself) give positive Ames tests for mutagenicity and the degree of mutagenicity can vary by orders of magnitude between diamines.
Currently, researchers at NASA-Lewis have developed a polyimide resin called AMB-21 which replaces MDA with 2,2-bis(4-[4-aminophenoxyl]phenyl)propane (BAPP), a non-toxic, non-carcinogenic monomer. AMB-21 has further benefit in that it may be formed into composite components by using resin transfer molding (RTM). RTM fabrication techniques fall into the category of "low-cost" composite processing, since RTM can cut the manufacturing costs by up to 50%. Unfortunately, AMB-21 has a glass transition temperature (after post-cure) of only 285.degree. C., which also falls short of the PMR-15 goal to achieve a 316.degree. C. composite use-temperature.